Some kind of ignition apparatus has IGBT (insulated gate bipolar transistor), which is a switching semiconductor, therein to control a current through an ignition coil connected with a spark plug. A short circuit or a contact of a wiring for controlling the IGBT with a power supply wiring makes control signal sent from an ECU (electrical control unit) continue for a long period of time to keep turning on the IGBT. In this case, the IGBT generates much heat, so as to break itself, that is, to cause a short circuit between the collector and the emitter thereof. Then, the IGBT flows a current continuously through a primary winding of the ignition coil, so that the ignition coil overheats to break itself.
U.S. Pat. No. 5,664,550B (JP3216972B) discloses an apparatus comprising an IGBT, a control circuit generating a control signal for the IGBT and a heat detection circuit, which detects a predetermined high level of heat generation at the IGBT. When the heat detection circuit detects the predetermined high level of heat, the detection circuit turns off the IGBT automatically. When a temperature of the IGBT falls down to the predetermined level again, a latch circuit continues to keep the control signal turned off to prevent the current through the IGBT from flowing without sufficient fall of the temperature in the IGBT. The IGBT keeps turned off until the control circuit determines to turn on the IGBT again.
Further, in the above apparatus, another IGBT takes out a little portion of the current through the ignition coil to detect the current value through the IGBT controlling the current through the ignition coil. Then a current limiting circuit controls a gate voltage of the IGBTs based on the current value. Thus, the current through the ignition coil settles at a predetermined level.
However, sometimes the heat generation at the ignition coil precedes the heat generation at the IGBTs due to high ambient temperatures and large resistance of the primary winding of the ignition coil is relatively large. In this case, the heat generation increases the resistance of the coil, so that the current is determined not by the current limiting circuit but by the resistance of the coil. Thus, the IGBTs operate in a saturation region, and the heat generation at the IGBTs is relatively small, without making the temperature of the die locating the IGBTs high. Accordingly, the heat detection circuit fails to turn off the IGBTs, so that the current keeps flowing through the ignition coil, and the ignition coil breaks by the heat generation thereof.